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MICROSTRUCTURAL AND TRIBOLOGICAL PROPERTIES OF
ULTRA FINE GRAINED HYBRID COMPOSITE PRODUCED BY
FRICTION STIR PROCESSING S. Soleymani*, A. Abdollah-Zadeh**,
S.A. Alidokht***
Department of Materials Engineering, Tarbiat Modares
University,
P.O. Box 14115-143, Tehran, Iran
*e-mail: [email protected]
**e-mail: [email protected]
***e-mail: [email protected]
Abstract. Ultra fine grained (UFG) Al5083 based hybrid composite
reinforced by SiC and MoS2 particles was prepared by Friction Stir
Processing (FSP). A constant tool travel speed of 50 mm/min and
different rotation speeds from 630 up to 1600 rpm were used. The
microstructure of the Al based composite were investigated and
compared to base metal and FSPed samples. It was found that
although FSP resulted in decreasing the mean grain size of the base
metal to about 2 µm, the addition of reinforcing particles in
microstructure led to the more decrease in grain size of the alloy.
An ultra fine grained hybrid composite with 500 nm grain size was
obtained through rotation speed of 1250 rpm. Moreover, this hybrid
composite showed the highest wear resistance and hardness in
comparison to all samples due to the modification of microstructure
and the addition of particles. 1. Introduction The plastic
deformation behaviors of Ultra Fine Grained (UFG) materials have
drawn tremendous interest due to the scientific and technological
importance of the problem. The mechanical properties of commercial
metals can be dramatically enhanced by extensive grain refinement,
thus offering new opportunities to exploit some attractive physical
properties in metals having a reasonably high strength for
structural applications. Fundamental research works on grain
refinement by intense plasticity published in recent years focussed
on the mechanisms that effectively reduce grain size down to
sub-micrometer in metals. In order to convert a coarse grained
solid into a material with ultra fine grains, it is necessary to
impose an exceptionally high strain in order to introduce a high
density of dislocations which, in turn, re-arrange to form an array
of grain boundaries with increase in strain [1, 2].
In contrast to conventional production methods, the severe
plastic deformation (SPD) techniques such as Friction Stir
Processing (FSP) are processes, where extremely high strains are
imposed at relatively low temperatures and can lead to equiaxed
microstructure and high angle grain boundary misorientation.
Materials processed by FSP have shown superior mechanical
properties such as high strength, excellent fatigue life, high
toughness and low temperature superplasticity [3, 4].
FSP, which was developed based on the principle of friction stir
welding (FSW), is remarkably simple. A rotating tool with a pin and
shoulder is inserted into a single piece of material and results in
significant microstructural changes in the processed zone, due
to
Materials Physics and Mechanics 17 (2013) 6-10 Received: October
14, 2012
© 2013, Institute of Problems of Mechanical Engineering
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intense plastic deformation. FSP has been proved to be an
effective way to refine the microstructure of aluminum alloys, and
thereby improve the mechanical properties [5, 6]. Previous
investigations [7, 8] have indicated that a fine grain structure
affects the tribological properties of surface layer. Prasada et
al. [9, 10] reported that the grain refinement leads to the
improvement of wear resistance and load bearing capacity of Al-7Si
alloy. Chandrashekharaiah and Kori [11] also reported similar
results for different Al-based alloys. One of the major problems
associated with Al5083 like other aluminum alloys, is their
relatively poor wear resistance which limits their tribological
performance [12].
The present work has been undertaken to develop ultra fine
grained Al alloy by FSP to examine its microstructure and
mechanical properties. An important objective of this work is to
identify and quantify interrelationships between the rotation speed
of FSP tool and mechanical behavior of FSPed Al alloy and UFG
aluminum based composite processed by FSP. 2. Experimental
procedure Commercially Al5083 rolled plates of 3 mm thickness with
a nominal composition of 4.3Mg–0.68Mn–0.15Si–bal. Al (in wt pct)
was the base material. Powders of SiC and MoS2 were mixed at weight
ratio of 2 to 1 and used as the reinforcements. MoS2 particles with
99.9 % purity and 5 µm average size used in this study as
lubricating material due to its lamellar morphology. SiC particles
with 99.9 % purity and 5 µm average particle size, was also used as
hard reinforcing material.
A tool made of steel H-13 with a shoulder of 20 mm diameter and
a pin of 6 mm diagonal length and 2.8 mm height with a tilt angle
of 3º was used to apply FSP. The mixture of reinforcing particles
was packed in a groove of 2 mm depth and 0.65 mm width machined out
on the Al plate.
Samples divided in two groups, namely, FSPed samples without
reinforcing powder and composite samples with the mixture of
reinforcements. Both groups were subjected to one FSP pass along
the same direction with travel speed of 50 mm/min and different
rotation speeds from 630 to 1600 rpm in room temperature. In order
to measure the temperature of the center of the processed line of
samples, a thermocouple with the accuracy of 5 oC were put into a
hole under the plates and the variations of temperature were
recorded.
Microstructural observations were performed on specimens by
transmission electron microscopy (TEM). The micro-hardness of the
surface composite layers was measured using 200 gr force. The wear
behavior of the specimens was evaluated by using a pin-on-disk
tester after 4000 m sliding in air at room temperature. Pin
specimens with 5 mm diameter were cut from the center of surface of
as-processed samples and ground on emery paper up to 320 grade.
Discs made of AISI D3 steel with hardness of 58 HRc used as
counterpart. The wear tests were carried out at normal loads of 5
KN and the rotation speed of 60 rpm. The wear weight loss was
measured with an accuracy of ±0.01 mg. 3. Results and Discussions
Figure 1 illustrates the variations of temperature of the center of
processed line for the samples FSPed at 630 to 1600 rpm. As can be
observed in this figure, increasing the rotation speed results in
increasing the heat input and hence the material’s temperature from
370 oC for 630 rpm to 575 and 595 oC for 1250 and 1600 rpm
respectively. Therefore, the rotating tool may provide the
activation energy for the occurrence of restoration mechanisms in
the Al alloy and can result in changing its microstructural and
mechanical characteristics. For this reason, the microstructure,
hardness and wear properties of the alloy should be
investigated.
Figure 2(a) shows the variations of grain size of base metal and
samples processed at 630 to 1600 rpm rotation speed. The results
indicated that FSP has led to the grain refinement comparing to
base metal. Moreover, increasing the rotation speed has resulted in
decreasing
7Microstructural and tribological properties of ultra fine
grained hybrid composite ...
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the grain size of processed zone. It can be mentioned that with
increasing the rotation speed from 1000 rpm, the mean size of
grains suddenly dropped to its minimum at 1250 rpm. It may be
attributed to the occurrence of different restoration mechanism
with the increase of heat input. Previous investigations [13, 14]
have indicated that in FSP/W, a continuous dynamic
recrystallization phenomenon can occur due to the mechanical action
of the tool pin and the frictional heat produced. This phenomenon
can lead to intensive microstructural refinement. However, with
increasing the rotation speed, the mean grain size increases again.
Increasing the rotation speed from 1250 up to 1600 rpm has probably
led to occurrence of more dynamic recrystallization and grain
growth due to higher heat input.
Fig. 1. Variations of temperature of the center of processed
line of samples produced by different rotation speeds.
(a) (b)
Fig. 2. Variations of mean grain size of (a) base metal and FSP
samples processed at the
rotation speed of 630 to 1600 rpm; (b) Composite samples
produced at the rotation speed of 630 to 1600 rpm.
Figure 2(b) shows the variations of grain size of composite
samples produced at 630 to
1600 rpm rotation speed. As can be seen in this figure, FSP has
led to the more grain refinement of composite samples in comparison
to FSPed specimens. This implies that the existence of reinforcing
particles can lead to the more grain refinement. However, the trend
of the variations of grain size is as same as for FSPed samples and
confirms the change in the restoration mechanism with increasing
the heat input due to the increasing rotation speed. Nevertheless,
the existence of reinforcing particles has resulted in decreasing
the mean grain
8 S. Soleymani, A. Abdollah-Zadeh, S.A. Alidokht
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size of composite samples in comparison to FSP specimens. The
results also indicated that increasing the rotation speed up to
1250 rpm, resulted in the formation of ultra fine grains in the
microstructure of produced composite material due to the occurrence
of restoration mechanisms along with the existence of reinforcing
particles.
Figure 3 shows the variations of the mean hardness of base metal
and samples processed at 630 to 1600 rpm rotation speed. As can be
seen in this figure, FSP has led to the increase in the hardness of
samples in comparison to as-received material. Furthermore,
increasing the rotation speed has resulted in more increase in
hardness of samples. These results are in good agreement with
microstructural observations and may be attributed to the increase
in yield strength of material owing to microstructural refinement
in accordance with Hall-Petch relation [5, 6]. As the rotation
speed increases from 1250 to 1600 rpm, the hardness drops due to
the increase in the grain size and heat input.
(a) (b)
Fig. 3. Variations of mean Vickers Hardness (HV) of processed
zone of (a) base metal and FSP samples processed at the rotation
speed of 630 to 1600 rpm; (b) Composite samples
produced at the rotation speed of 630 to 1600 rpm.
(a) (b)
Fig. 4. Variations of weight loss of (a) base metal and FSP
samples processed at the rotation speed of 630 to 1600 rpm; (b)
Composite samples produced at the rotation speed of 630 to
1600 rpm.
Figure 4(a) shows wear weight loss of base metal and samples
FSPed at different rotation speeds. The results indicate that FSP
has led to the decrease in wear weight loss compared with
as-received metal and hence improving wear resistance. This can be
attributed to the increase in hardness due to the microstructural
refinement. Besides, as can be seen in
9Microstructural and tribological properties of ultra fine
grained hybrid composite ...
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this figure, wear weight loss of samples decreases with the
increase in the rotation speed of FSP tool. This observation is in
good agreement with the trend observed for the hardness of the
samples (Fig. 3). Increasing the hardness of metal can lead to the
occurrence of less plastic deformation during sliding [9, 10]. The
results indicate that FSP can lead to the improvement of wear
resistance as well as hardness especially for higher rotation
speeds up to 1250 rpm due to the improvement of microstructural
characteristics.
Figure 4(b) illustrates wear weight loss of composite samples
produced by different rotation speeds. The observed trends for
these samples were similar to those for FSP specimens. As can be
seen in both Figs. 4(a) and (b), there is a sharp decrease in the
amount of weight loss of samples processed at 1250 rpm. This
observation is in accordance with the results obtained from the
hardness and mean grain size shown in Figs. 2 and 3 and confirms
the occurrence of different restoration mechanism occurred in the
rotation speed of 1250 rpm. 4. Conclusions In the present
investigation, an attempt has been made to study the
microstructural and mechanical properties of Al based composites
and compare it to base metal and FSPed samples. The obtained
results can be summarized as follows:
1- FSP resulted in grain refinement of the alloy in comparison
to base metal. Moreover, increasing the rotation speed led to the
more decrease in the size of grains and this factor dropped sharply
to 2 µm in 1250 rpm owing to the restoration phenomenon. The same
trends were observed for produced composite samples and the
rotation speed of 1250 rpm obtained ultra fine grained
microstructure with 0.5 µm mean grain size.
2- FSP has led to the increase in the hardness of the alloy.
Increasing the rotation speed also led to the improvement of
hardness and maximum value were obtained in 1250 rpm. These can be
attributed to the microstructural refinement.
3- FSP resulted in the improvement of wear resistance of the
alloy. Moreover, increasing the rotation speed led to the
improvement of wear resistance. Similar trend were also observed
for composite samples and confirmed the improving effect of higher
rotation speed on the wear resistance of the alloy. This can be due
to the increase in hardness and microstructural refinement. Ultra
fine grained microstructure which was obtained by 1250 rpm, showed
the highest wear resistance amongst all samples due to the highest
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10 S. Soleymani, A. Abdollah-Zadeh, S.A. Alidokht